Well, the advantages and this is pure speculation is:

** Less XP advancement, except the Wizard?

** Only One attribute needed to get class. But with, well... I use 3d6 PaR (Placed as Rolled), but other folks might use 4d6RL (remove lowest) and arrange attributes. So, that takes the fun from that.

** No Alignment restrictions: Except Thief, but who cares about them, right?

** Now that I look at it, the base classes just there for convenience. If you play traditionally, it be 3d6 PaR and if your attributes or low-average, but you still have a Str 9. You can opt for Fighter and still play. But this days some (most?) GM allow for more variety (The varied Methods, noted from AD&D 1E, which caused me serious mental problems, transitioning from basic D&D blue book). I play this game with 3d6 PaR because the rules (especially for Character creation) are only restrictive while using Method I, and based on the old OD&D ruleset.

Granted, this is just a theory and it best get a more official opinion of the question.



 

Also keep in mind that the four principal classes have but one primary attribute as opposed to two (like all the subclasses). So, if that primary attribute (ST for fighters, IN for magicians, WS for clerics, DX for thieves) is 16 or greater, a 10% XP bonus is gained.